Screen printing machine

ABSTRACT

A screen printing machine has a bearing supporting a first arm structure for rotation about a vertical axis, the first arm structure comprising first arms equiangularly distributed about the vertical axis and extending radially outwardly of the vertical axis. The first arms each carry, at a free outer end thereof, a printing screen support platen. A second arm structure has second arms equiangularly distributed about the vertical axis and extending radially outwardly of the vertical axis, the second arms each carrying, at a free outer end thereof, a squeegee mechanism and a printing screen support for holding printing screen below the squeegee mechanism. An indexing drive rotates the first arm structure stepwise around the vertical axis so as to position the platens in succession in registry with the squeegee mechanisms; and interengageable pairs of connectors are provided on the first and second arms for forcibly connecting the first arms to the second arms, to counteract bending of the arms during the operation of the squeegee mechanisms.

This application is a continuation of application Ser. No. 08/138,074,filed Oct. 20, 1993 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to screen printing machines and is useful,in particular, for rotary machines for printing on T-shirts and thelike.

2. Description of the Related Art

It has previously been proposed to construct a screen printing machine,for printing T-shirts, having a first set of arms extending radiallyoutwardly of a central, vertical axis and carrying, at their free ends,platens for supporting T-shirts, with a second set of radially extendingarms located above the first set. An indexing mechanism rotates thefirst set of arms, in a stepped manner, so that each of the first armsis brought, in succession, beneath each of the second arms.

The free ends of the second arms carry printing screen holders forgripping and retaining printing screens, together with squeegeemechanisms located above the printing screen holders for performing asqueegee action on the printing screens.

In operation of such a device, the T-shirts are fitted over the platensand the first set of arms are then rotated to bring each T-shirt beneatheach of the printing screens, in succession. The rotation of the firstset of arms is interrupted, after each step in the rotation, and thefirst set of arms are then raised so as to press the T-shirts againstthe undersides of the screens, whereupon the squeegee mechanisms areoperated.

BRIEF SUMMARY OF THE INVENTION

It is an objection of the present invention to provide a novel andimproved screen printing machine of the above-described type providedwith connectors operable to simply and temporarily connect the first andsecond arms to counteract bending of the arms.

According to the present invention, there is provided a screen printingmachine comprising the bearing arrangements supporting a first armstructure for rotating about a vertical axis, the first arm structurecomprising first arms equiangularly distributed about the vertical axisand extending radially outwardly of the vertical axis. Each of the firstarms carries, at a free outer end thereof, a printing screen supportplaten. The machine includes a second arm structure comprising secondarms equiangularly distributed above the vertical axis and extendingradially outwardly of the vertical axis, the second arms each carrying,at a free outer end thereof, a squeegee mechanism and a printing screensupport for holding a printing screen below the squeegee mechanism.

An indexing mechanism rotates the first arm structure stepwise aroundthe vertical axis so as to position the platens in succession inregistry with the squeegee mechanisms.

Interengageable pairs of connectors provided of the first and secondarms, at locations adjacent and radially inward of the printing screensupports, are operable to forcibly connect the first arms to the secondarms, and thereby to counteract bending of the arms, during theoperation of the squeegee mechanisms.

The squeegee mechanisms each comprise a squeegee for sliding contactwith a printing screen and a carriage carrying the squeegee for movementto and fro over the printing screen. The squeegee has a pair of mutuallyangularly displaced squeegee blades and a pivotal connection is providedbetween the squeegee and the carriage for permitting pivotation of thesqueegee about an axis parallel to the squeegee blades for bringing thesqueegee blades alternately into contact with the printing screen duringthe to and fro moments, respectively, of the carriage. A piston andcylinder device mounted on the carriage is connected to the carriage andthe squeegee for effecting pivotation of the squeegee about the axis,and an adjustable stop mechanism adjustably limits the pivotation of thesqueegee by the piston and cylinder device.

A vertically adjustable connection is provided between the squeegee andthe carriage and comprises a cylindrical threaded member extendingdownwardly from the carriage, a manually rotatable adjustment ringmember in threaded engagement with the cylindrical threaded member, anda slide member carrying the squeegee, the slide member being supportedon the ring member for vertical displacement upon rotation of the ringmember about the cylindrical threaded member.

A piston rod and cylinder device has a piston rod connected to thesqueegee carriage for effecting to and fro movement of the squeegee overthe printing screen and an air brake is provided for braking suchmovement. The air brake comprises a pair of brake members formed withcylindrically curved braking surfaces and air passages for supplyingcompressed air to the brake members so as to urge the braking surfacesinto frictional braking contact with opposite sides of the piston rod.

The machine includes a lifting device for raising the first armstructure towards the second arm structure, a stop for limiting theupward movement of the first arm structure towards the second armstructure and a height adjustment mechanism for adjustably raising andlowering the stop and thereby adjusting the limit of the upward movementof said first arm structure. The height adjustment mechanism includes amanually adjustable member accessible from the exterior of the machine.

A shock absorber is connected to the first arm structure for damping thestepwise rotation of said first arm structure by the indexing drive andcomprises an elongate piston slidable to and fro in a cylinder and aplurality of fluid outlet openings spaced apart along the cylinder forpermitting hydraulic fluid to escape from the cylinder in response tosliding of the piston within the cylinder towards one end of thecylinder. The piston is dimensioned to successively block the outletopenings as the piston slides into the cylinders.

The indexing mechanism comprises an indexing arm mounted for pivotationabout the vertical axis, a piston and cylinder device connected betweenthe indexing arm and the machine frame for effecting the pivotation ofthe indexing arm, a connector member at a free end of the indexing armand an actuator for raising the connector member into drive transmittingengagement with successive ones of the first arms.

The printing screen supports each comprise a screen holder for holding aprinting screen, a support structure carrying the screen holder, and anadjustment mechanism between the support structure and the respectiveone of the second arms, the adjustment mechanism being operable toadjustably displace the support structure longitudinally andtransversely of its second arm.

The screen holder comprises a pair of spaced screen supports forsupporting opposite edges of the printing screen and the supportstructure is provided with adjustable connectors between the screensupports and the support structure to allow the screen supports to beadjusted in position relative to one another and to the supportstructure.

The second arms each comprising first arm member extending radiallyoutwardly of the vertical axis, a second arm member extending radiallyoutwardly of the vertical axis and downwardly and outwardly inclinedtowards the first arm member, releasable fasteners for securing thesecond arm member to the first arm member and an adjustment mechanismfor adjusting the second arm member in position along the first armmember while the fasteners are released.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, advantages and objects of the present invention willbe more readily apparent to those skilled in the art from the followingdescription thereof when taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 shows a plan view of a rotary screen printing machine accordingto the present invention;

FIG. 2 shows a view taken in vertical cross-section through the machineof FIG. 1;

FIG. 3 shows a view in side elevation of an arm forming part of thescreen printing machine of FIG. 1;

FIG. 4 shows a view taken in cross-section along the line 4--4 of FIG.3;

FIG. 5 shows a view in perspective of parts of a squeegee mechanism ofthe machine of FIG. 1;

FIG. 6 shows a view in side elevation of parts of the squeegee mechanismof FIG. 5;

FIG. 7 shows a view in end elevation of the squeegee mechanism parts ofFIG. 6 and of part of an arm of the printing machine of FIG. 1;

FIG. 8 shows a view taken in cross-section along the line 8--8 of FIG.9;

FIG. 9 shows a view taken in vertical cross-section through part of anarm of the machine of FIG. 1;

FIG. 10 shows a view in perspective of an air brake;

FIG. 11 shows a view in front elevation of the air brake of FIG. 10;

FIG. 12 shows a broken-away view of an indexing mechanism and otherparts of the machine of FIG. 1;

FIG. 13 shows a broken-away view in side elevation of the indexingmechanism of FIG. 12 and other parts of the machine of FIG. 1;

FIG. 13A shows a view taken in vertical cross-section through aconnector device forming part of the apparatus of FIGS. 12 and 13;

FIG. 14 shows a view in longitudinal cross-section through a shockabsorber forming part of the indexing mechanism of FIG. 12;

FIG. 15 shows a view in side elevation of parts of a fitting device ofthe machine of FIG. 1;

FIG. 16 shows a view, taken in the direction of arrow A of FIG. 15;

FIG. 17 shows a view in perspective of a printing screen holder andassociated adjustment mechanism;

FIG. 18 shows a view in perspective of other parts of the screen holderadjustment mechanism on a broken-away end of one of the arms of themachine of FIG. 1; and

FIG. 19 shows a broken-away view, in perspective, of a connectorarrangement forming part of the machine of FIG. 1.

THE PREFERRED EMBODIMENT

FIGS. 1 and 2 show a rotary printing machine indicated generally byreference numeral 10, which has a first or lower arm structure indicatedgenerally reference numeral 12 and a second or upper arm structureindicated generally by reference numeral 14.

The second arm structure 14 is stationary and is mounted above the firstarm structure 12 on a central, vertical cylindrical column or support16, which, in turn, is fixably connected to a machine frame indicatedgenerally by reference numeral 18, said column 16 and said machine frame18 forming a support structure which serves to support the entireprinting machine 10 on a floor 19.

The first arm structure 12 comprises twelve generally horizontallyextending first arms, two of which are indicated by reference numerals20, which are equiangularly spaced about the vertical longitudinal axisof the cylindrical support 16 and which extend radially outwardly ofthis vertical axis.

The first arm structure 12 is carried on a sleeve 22, which is co-axialwith the cylindrical support 12 and which is supported for movement toand fro, in the vertical direction, along the exterior of thecylindrical support 16 by means of rollers 24 in rolling engagement withthe outer surface of the cylindrical support 16.

The lower end of the sleeve 22 is provided with a radially outwardlyextending flange 26, which is carried on the upper ends of two pistonrods 28.

The piston rods 28 form parts of a lifting device connected between thefirst arm structure 12 and the support structure and operable todisplace the former to and from a raised position, the lifting devicecomprising vertically acting piston and cylinder devices indicatedgenerally by reference numerals 30, the cylinders of which are fixed tothe machine frame 18.

A horizontal bar 32 connects the lower ends of the piston rods 28, andextends below a stop in the form of a pad 34 mounted on the supportstructure for limiting the upward movement of the first arm structure 12towards the second arm structure 14. The arrangement is such that, onexpansion of the piston and cylinder devices 30 in order to raise thesleeve 22 and, thereby, the entire first arm structure 12, into a raisedposition relative to the cylindrical support 16 and the second armstructure 14, the upward movement of the first arm structure 12 islimited by abutment of the bar 32 against the pad 34.

The first arm structure 12 is supported on the sleeve 22 by means of abearing arrangement co-axial with the vertical support 16 and comprisingbearings 36 so as to be rotatable about the common vertical axis of thesleeve 22 and the cylindrical support 16.

The second arm structure 14 comprises twelve arms 40, which areequiangularly distributed around, and extend radially of, the verticalaxis of the cylindrical support 16.

At their free outer ends, the first arms 20 each carry a printing screensupport platen 42, whereas the second arms 40 each carry, at their outerfree end, a printing screen holder indicated generally reference numeral44, for holding a printing screen 46, and a squeegee mechanism indicatedgenerally by reference numeral 48, the printing screens 46 thus beingheld below the squeegee mechanisms.

The second arms 40 each comprise a substantially horizontally extendingfirst arm member 50, which is connected at its inner end to thecylindrical support 16, and an inclined second arm member 52, whichextends above the respective first arm member 50 and which is radiallyoutwardly and downwardly inclined to the respective first arm member 50,the inner end of the second arm member 52 being connected to the top ofthe cylindrical support 16.

More particularly, as illustrated in FIGS. 3 and 4, the arm member 50 isin the form of an extrusion, which has, at opposite sides, upperlongitudinal recesses 54 containing square nuts 56, which are inthreaded engagement with bolts 58 extending through slots in theextrusion 50 and in opposite side walls 60 of the arm member 52. Bytightening the bolts 58, the side walls 60 can be clamped to oppositesides of the extrusion 50.

The extrusion 50 is also formed with a pair of opposed raceways 62which, as illustrated in FIG. 7, form a track for rollers 64.

The rollers 64 carry the squeegee mechanism 48, which includes asqueegee carriage indicated generally reference numeral 66 in FIGS. 5and 6, the rollers 64 being omitted from FIG. 5 to facilitate theillustration.

The raceways 62, the rollers 64 and the carriage 66 form a suspensionarrangement supporting a squeegee 94 (FIG. 5) for movement to and froover the printing screen 46, as described in greater detail below.

The squeegee carriage 66 is formed by a horizontal plate 68, a verticalplate 70 depending from the plate 68 and a sleeve 72, which also dependsfrom the plate 68.

The lower end of the sleeve 72 is formed with a screw thread 74, whichis in threaded engagement with a corresponding internal screw thread inan adjustment ring 76. The adjustment ring 76 contains an inner ring 78,which extends around the sleeve 72 and which is retained in an annularrecess in the adjustment ring 76 by means of retaining screws 80 andwashers 82.

A cylindrical yoke 84 has an upper end slidably received in the sleeve72, and is supported by means of a transverse pin 86, opposite ends ofwhich engage in the ring 78. The pin 86 is a slide member slidable alongvertical slots, one of which is indicated by reference numeral 87 andwhich extend through opposite sides of the sleeve 72. By rotation of theadjustment ring 76 relative to the sleeve 72, the inner ring 78 and,therewith, the yoke 84 are vertically adjusted in position relative tothe sleeve 72.

A rocker member 88 is pivotally supported in the yoke 84 by means of apivot pin 90 so as to be pivotable to and fro about the pin 90. Therocker member 88 is connected by a connecting bracket 92 to a squeegeeindicated generally by reference numeral 94, which has a pair ofmutually angularly disposed squeegee blades 96.

The pivot pin 90 forms a pivotal connection between the squeegee 94 andthe carriage 66, permitting pivotation of the squeegee about an axis,i.e. the pivot axis of the pivot pin 90, parallel to the squeegee blades96.

The squeegee carriage vertical plate 70 has a horizontally rearwardlyextending arm 98, to which a pneumatic piston and cylinder deviceindicated generally by reference numeral 100 is connected by means of aneye-bolt 102 and a pivot pin 104. The piston and cylinder device 100 hasa piston rod 106, the free end of which is pivotally connected to aforked part 108 of the rocker member 88 and, thus, to the squeegee 94.

An elongate member in the form of a slide rod 110 is connected at itsopposite ends, by metal bars 112, to the piston rod 106 so as to bemovable to and fro therewith. The rod 110 carries a pair of stop memberscomprising adjustment nuts 114, which are in threaded engagement withthe rod 110, and the rod 110 extends through a circular opening in anabutment 116, which is fixed to cylinder 118 of the piston and cylinderdevice 100. By rotatably adjusting the nuts 114 in position along thelength of the rod 110, the limits of the to and fro movement of thepiston 106 can be adjusted and, thus, the angle through which thesqueegee 94 is pivoted by the operation of the piston and cylinderdevice 100 is likewise adjusted.

The squeegee carriage 66 is driven to and fro along the arm member 50 bymeans of a piston and cylinder device indicated generally by referencenumeral 120 in FIG. 9. The piston and cylinder device 120 has a pistonrod 122, which is secured by a knurled retaining bolt 124 to a bracket126 which depends from the rear end of the squeegee carriage horizontalplate 68.

The piston rod 122 extends through an air brake which is indicatedgenerally by reference numeral 128 and which is shown in greater detailin FIGS. 10 and 11.

The air brake 128 has a cylindrical housing 130, which is provided atopposite sides with circular openings, one of which is indicated byreference numeral 132, through which the piston rod 122 extends.

Opposite ends of the cylindrical housing 130 are closed by a pair ofsolid cylindrical metal plugs 134 (FIG. 11), which are formed with airpassages 136. Each air passage 136 communicates through a circularopening 138 in the bottom of the cylindrical housing 130 with acompressed air supply tube 140, and also communicates with a cylinderspace 142 within the cylindrical housing 130 between the two cylindricalplugs 134.

The cylinder space 142 contains a pair of pistons 144, which areprovided at opposite sides of the piston rod 122 and which are formedwith opposed semi-cylindrical recesses 146 containing cylindricallycurved brake members 148, which are in sliding contact with oppositesides of the piston rod 122. The brake members 148 are made ofgraphite-impregnated nylon.

O-ring seals 150 and 152 are provided between the metal plugs 134 andthe pistons 144, on one hand, and the internal cylindrical surface ofthe cylindrical housing 130, on the other hand.

In operation of this air brake, compressed air is supplied through theair passages 136 and acts on the pistons 144 so as to press the brakemembers 148 against the piston rod 122 and, thereby, to brake themovement of the squeegee carriage 66 as the latter approaches theopposite ends of its path of movement.

Reference is now made to FIGS. 12 through 14, which illustratecomponents of an indexing mechanism connected between the first armstructure 12 and the support structure comprising the machine frame 18for effecting the stepwise rotation of the first arm structure 12 aroundthe vertical axis of the cylindrical support 16 so as to position theplatens 42 in succession in registry with the squeegee mechanisms 48.

The indexing mechanism comprises an indexing arm 160, which is pivotableto and fro about the cylindrical column 16.

A pneumatic piston and cylinder device 164 is pivotally connected to themachine frame 18 and to the indexing arm 160 for effecting thepivotation of the latter.

The indexing arm 160 is provided, at its underside, with a downwardlyextending plate 162, carrying upper and lower rollers 164 and 166 whichare in rolling engagement with the upper and lower surfaces,respectively, of an arcuately curved support plate 168 mounted on themachine frame 18 for counteracting bending of the indexing arm 160.

At its free end, the indexing arm 160 carries a connector deviceindicated generally by reference numeral 170, which has a cylindricalhousing 172 containing a connector member in the form of a cylindricalsocket 174, which is provided at its top with a socket recess 176. Inresponse to a supply of compressed air to the connector 170, the socketmember 174 is displaced upwardly into driving engagement with acorrespondingly shaped pin 178 extending downwardly from the first arm20, as shown in FIG. 13.

Each of the first arms 20 of the first arm structure 12 is provided withone of the pins 178, so that the arms 20 can be drivingly engaged anddisplaced, in succession, by operation of the indexing mechanism.

As can be seen from FIG. 13A, the socket 174 forms a piston which isvertically slidable in the cylindrical housing 172 and which isconnected by a rod 173 to another piston 175, which is also verticallyslidable in the housing 172. The piston 175 is retained on the lower endof the rod 173 by a retaining bolt 177, and the rod 173 is slidablethrough a cylindrical block 179 which is fixedly positioned in thehousing 172. Pneumatic tubes 181 and 183 communicate with chambers 185and 187 at opposite sides of the block 179, through passages 189 in theblock 179, so that the vertical displacement of the socket 174 into endout of engagement with the pins 178 can be effected pneumatically.

The pivotation of the indexing arm 160 for drawing the first armstructure 12, and thus each step of the rotation of the first armstructure 12 by the indexing mechanism, are damped by means of a shockabsorber 180 which, as illustrated in FIG. 12, is connected between themachine frame 18 and an outer end portion of the indexing arm 160.

The construction of the shock absorber 180 is illustrated in greaterdetail in FIG. 14, from which it can be seen that the shock absorber 180has an elongate piston 182, which is slidable to and fro within an innercylinder 184, which fits snugly within an outer cylinder 186. Seals 187are provided between the inner and outer cylinders 184 and 186 atopposite ends thereof. One end of the inner cylinder 184 is closed by anend closure member 188, which is formed with an outlet opening in theform of a longitudinal hydraulic fluid passage 190 opening, at one end,into the interior of the inner cylinder 184 and, at its opposite end,into a transverse hydraulic fluid passage 192. The end closure 188 issecured in position in the end of the inner cylinder 184 by a pair ofgrub screws 194, inserted through openings in the inner and outercylinders 184 and 186 and in threaded engagement with the end closuremember 188.

The piston 182 has, along its entire length, a diameter whichcorresponds, with a slight clearance to allow the sliding of the piston182, to the internal diameter of the inner cylinder 184.

The external surface of the cylinder 184 is formed, along a portion ofthe length of the inner cylinder 184 adjacent the end containing the endclosure 188, with a flat 196, which provides a recess in the externalsurface of the inner cylinder 184 and, thus, which provides a fluidpassage extending longitudinally of the cylinders between the innercylinder 184 and the outer cylinder 186.

A plurality of transversely extending hydraulic fluid outlet openings198 are spaced-apart along the inner cylinder 184 and, when not blockedby the piston 182, provide communication between the interior of theinner cylinder 184 and the fluid passage formed by the flat 196, whichin turn communicates with an outlet 200 through the outer cylinder 186.

The arrangement is such that, as the indexing arm 160 is pivoted foradvancing the first arm structure 12 through each step of its rotation,the piston 182 is forced to the right, as viewed in the FIG. 14, by theindexing arm 160. During an initial portion of the movement of thepiston 182 into the inner cylinder 184, hydraulic fluid can flow fromthe interior of the inner cylinder 184 through all of the outletopenings 198 to the common outlet opening 200. As the piston 182continues to advance into the inner cylinder 184, the piston 182successively blocks the openings 198 and, thus, the outflow of hydraulicfluid through the opening 200 is progressively reduced until, eventuallythe piston 182 blocks the right-hand opening 198, as viewed in FIG. 14.When that occurs, hydraulic fluid remaining in the inner cylinder 184can escape only through the passage 190. Therefore, the movement of thepiston 182 into the inner cylinder 184, and likewise the pivotalmovement of the indexing arm 160, are subjected to a progressivelyincreasing damping force as the index arm 160 reaches the end of itspivotal stroke.

As described above with reference to FIG. 2, the upward displacement ofthe first arm structure 12 is limited by abutment of the crossbar 32against the underside of the stop pad 34. FIGS. 15 and 16 illustrate aheight adjustment mechanism for adjusting the height of the stop pad 34and, thereby, for adjusting the height to which the first arm structure12 can be raised by the piston and cylinder devices 30.

As shown in FIGS. 15 and 16, the stop pad 34 is provided with a verticalmetal plate 210, which extends upwardly from a horizontal plate 212 onthe underside of which the stop pad 34 is provided.

The plate 210 is vertically slidable between a pair of vertical slideplates 214, which extend downwardly from opposite sides of an elongateslide bar 216, which is secured to the underside of a horizontal beam218 forming part of the machine frame 18.

A horizontal pin 220 extends from opposite sides of the plate 210 and isvertically slidable along vertical slots 222, of which only one isshown, in the side plates 214. A pair of tension springs 224, extendingbetween opposite ends of the pin 220 and opposite ends of a further pin226 projecting from opposite sides of the slide bar 216, resilientlyurge the plate 210 in an upward direction.

The plate 210 is formed with an inclined upper edge 228, which abuts acorrespondingly inclined edge 230 of a wedge member 232, which isdisplaceable to and fro along the slide bar 216.

A threaded spindle 233, provided at the one end with a manuallyadjustable member comprising a knob 234, which is readily accessiblefrom the extension of the machine and is in threaded engagement with ablock 236 secured to a part of the machine frame 18. The other end ofthe spindle 232 is in threaded engagement with a tube 238, which issecured to a pneumatic piston and cylinder device 230, which has apiston rod 242 connected to the wedge 232.

By operation of the piston and cylinder device 230, the wedge 232 can bedisplaced to and fro in order to correspondingly displace the stop pad34 between an upper position and a lower position. The wedge 232 thuscontrols the locations of the raised and lowered positions of the firstarm structure 12 on operation of the piston and cylinder devices 30.

In addition, by rotation of the knob 234, the wedge 232 can be adjustedin position along the slide bar 216 so as to correspondingly adjust theupper and lower limits of the vertical movement of the first armstructure 12.

Referring now to FIGS. 17 and 18, which show the print screen holder 44and an adjustment mechanism, indicated generally by reference numeral240, for adjusting the print screen holder 44 in position relative toits second arm 40, it will be seen that the holder 44 comprises a pairof spaced, mutually opposed print screen supports in the form ofhorizontally extending, laterally open channel members 242. The channelmembers 242 carry the screen 46 between them as shown in FIG. 2 and areeach provided with a pair of pneumatically operably clamping devices 244for clamping the printing screen 46 in position relative to the channelmembers 242.

The channel members 242 are carried by vertical rods 246 extendingdownwardly from a support structure comprising a support frame which isformed by a pair of parallel, horizontally extending elongate members inthe form of rods 248 connected together at opposite ends by crossbars250 and 252. The rods 246 are adjustably secured to the rods 248 byadjustable connectors in the form of damping devices 253.

The crossbar 250 can be clamped to a bracket 254, carried on the end ofthe arm 40, by means of a clamping device 256.

The crossbar 252 is secured to the underside of the arm 40 by a pair ofretainers 258 and adjustably connected to the rods 248 by clampingdevices 259. The adjustment mechanism 240 includes a pair of adjustmentmembers in the form of spindles 260 and 262, provided with respectiveknobs 264 and in threaded engagement with respective supports 266 and268 extending from the bracket 254 and one side of the arm 240,respectively, and pivotally connected by connector members 270 to avertical pin 278 on one end of the crossbar 250.

The adjustment mechanism 240 also includes a further adjustment membercomprising a threaded spindle 280 which is pivotally connected, by aconnector member 282, to one end of one of the parallel rods 248, andextends in threaded engagement with a support bracket 284, which isfixed to the opposite side of the arm 40, the spindle 280 being providedwith an operating knob 286.

On release of the clamping devices 256 and 259, and by appropriateadjustment of the knobs 264 and 286, the print screen holder 44 can bedisplaced longitudinally and transversely of the arm 40, and can beadjusted angularly, in a substantially horizontal plane, relative to thearm 40.

Referring again to FIG. 3, it will be seen that the bracket 254 at theend of the arm member 50 is provided, at its top, with an adjustmentmechanism comprising an adjustment bolt 290, which is in threadedengagement with the bracket 254. The adjustment bolt 290 is inengagement with an outer end of the second arm member 52 so that, onloosening of the fasteners, comprising the bolts 58 and the nuts 56, therotation of the adjustment bolt 290 can be employed to displace thesecond arm member 52 along a part of the length of the first arm member50, for correspondingly adjusting the height of the latter, whereuponthe bolts 58 can again be tightened.

The first and second arms 20 and 40 are provided with interengageablepairs of connectors indicated generally by reference numerals 300 and302, at locations adjacent and radially inward of the printing screensupports 44. The connectors 300 and 302 are operable to forcibly connectthe first arms 20 to the second arms 40 on displacement of the first armstructure 12 into its raised position.

The connectors 300 each comprise a pneumatic piston and cylinder device304 having a downwardly extending piston rod 306 provided, at itslowermost end, with a flat disk-shaped head 308.

The connectors 302 each comprise a co-operating member in the form of abracket 310 secured to opposite sides of the respective first arm 20 andformed with opposed horizontal openings 312 for the passage of the head308 through the bracket 310 and, at the top of the bracket 310, a slot314 for the passage of the piston rod 306.

When the arms 20 come to rest beneath the arms 40, each connector head308 is located within the respective brackets 310, as illustrated inFIG. 19. The piston and cylinder devices 304 are then retracted toengage the tops of the heads 308 with the brackets 310 to forciblyconnect the arms 20 to the arms 40 and, thereby, to raise the first arms20 towards the second arms 40 so as to counteract any bending of thefirst arms 20 during the operation of the squeegee mechanisms 48.

As will be readily apparent to those skilled in the art, variousmodifications may be made in the above-described machine within thescope and spirit of the appended claims.

I claim:
 1. A screen printing machine, comprising:a support structurefor supporting said machine on a floor; said support structurecomprising a vertical support having a vertical axis; a first armstructure; a bearing arrangement co-axial with said vertical cylindricalsupport and supporting said first arm structure for rotation about thevertical axis; said first arm structure comprising a plurality of firstarms equiangularly distributed about the vertical axis; said first armsextending radially outwardly of said vertical axis and having free outerends; printing screen support platens carried on said free outer ends ofsaid first arms; a second arm structure mounted on said vertical supportabove said first arm structure; said second arm structure comprising aplurality of second arms equiangularly distributed about said verticalaxis; said second arms extending radially outwardly of said verticalaxis above said first arm structure and having free outer ends; saidsecond arms carrying, at said free outer ends thereof, squeegeemechanisms and printing screen supports for holding printing screensbelow said squeegee mechanisms; an indexing mechanism connected betweensaid first arm structure and said support structure and operable torotate said first arm structure stepwise around the vertical axis so asto position said printing screen support platens in succession inregistry with said squeegee mechanisms; a lifting device connectedbetween said first arm structure and said support structure and operableto displace the former to and from a raised position; andinterengageable pairs of connectors provided on said first and secondarms, respectively, at locations adjacent and radially inward of saidprinting screen supports and operable to forcibly connect said firstarms to said second arms on displacement of said first arm structureinto the raised position, and thereby to counteract bending of said armsduring the operation of said squeegee mechanisms.
 2. A screen printingmachine as claimed in claim 1, wherein said pairs of connectors eachcomprise a piston and cylinder device carried on one of said second armsand having a downwardly extending piston rod and a head carded by alowermost end of said piston rod, and a co-operating member on one ofsaid first arms, said co-operating member having a horizontal openingfor movement of said head into and beyond said co-operating memberduring the rotation of said first arm structure and a slot extendingupwardly from said opening for receiving said piston rod, said headbeing engageable with said co-operating member by retraction of saidpiston and cylinder device.
 3. A screen printing machine, comprising:asupport structure for supporting said machine on a floor; said supportstructure comprising a vertical support having a vertical axis; a firstarm structure; a bearing arrangement co-axial with said verticalcylindrical support and supporting said first arm structure for rotationabout the vertical axis; said first arm structure comprising a pluralityof first arms equiangularly distributed about the vertical axis; saidfirst arms extending radially outwardly of said vertical axis and havingfree outer ends; printing screen support platens carried on said freeouter ends of said first arms; a second arm structure mounted on saidvertical support above said first arm structure; said second armstructure comprising a plurality of second arms equiangularlydistributed about said vertical axis; said second arms extendingradially outwardly of said vertical axis above said first arm structureand having free outer ends; said second arms carrying, at said freeouter ends thereof, squeegee mechanisms and printing screen supports forholding printing screens below said squeegee mechanisms; an indexingmechanism connected between said first arm structure and said supportstructure and operable to rotate said first arm structure stepwisearound the vertical axis so as to position said printing screen supportplatens in succession in registry with said squeegee mechanisms; alifting device connected between said first arm structure and saidsupport structure and operable to displace the former to and from araised position; suspension arrangements supporting said squeegeemechanisms from said second arms for movement to and fro over theprinting screens; piston and cylinder devices connected between saidsuspension arrangements and said first arms for effecting the to and formovement of said squeegee mechanisms; and air brakes for braking the toand fro movement of said squeegee mechanisms, said piston and cylinderdevices including piston rods extending through said air brakes and saidair brakes each comprising a pair of brake members formed withcylindrically curved braking surfaces at opposite sides of a respectiveone of said piston rods and air passages for supplying compressed air tosaid brake members so as to urge said braking surfaces into frictionalbraking contact with the opposite sides of respective one of said pistonrods.
 4. A screen printing machine, comprising:a support structure forsupporting said machine on a floor; said support structure comprising avertical support having a vertical axis; a first arm structure; abearing arrangement co-axial with said vertical cylindrical support andsupporting said first arm structure for rotation about the verticalaxis; said first arm structure comprising a plurality of first armsequiangularly distributed about the vertical axis; said first armsextending radially outwardly of said vertical axis and having free outerends; printing screen support platens carried on said free outer ends ofsaid first arms; a second arm structure mounted on said vertical supportabove said first arm structure; said second arm structure comprising aplurality of second arms equiangularly distributed about said verticalaxis; said second arms extending radially outwardly of said verticalaxis above said first arm structure and having free outer ends; saidsecond arms carrying, at said free outer ends thereof, squeegeemechanisms and printing screen supports for holding printing screensbelow said squeegee mechanisms; an indexing mechanism connected betweensaid first arm structure and said support structure and operable torotate said first arm structure stepwise around the vertical axis so asto position said printing screen support platens in succession inregistry with said squeegee mechanisms; a lifting device connectedbetween said first arm structure and said support structure and operableto displace the former to and from a raised position; interengageablepairs of connectors provided on said first and second arms,respectively, at locations adjacent and radially inward of said printingscreen supports and operable to forcibly connect said first arms to saidsecond arms on displacement of said first arm structure into the raisedposition, and thereby to counteract bending of said arms during theoperation of said squeegee mechanisms; a stop mounted on said supportstructure for limiting the upward movement of said first arm structuretowards said second arm structure; a height adjustment mechanismconnected to said stop for adjustably raising and lowering said stop andthereby adjusting the limit of the upward movement of said first armstructure; and said height adjustment mechanism including a manuallyadjustable member, which is readily accessible from the exterior of saidscreen printing machine, for operating said height adjustment mechanism.5. A screen printing machine as claimed in claim 4, wherein said heightadjustment mechanism further comprises a piston and cylinder deviceoperable to displace said stop between raised and lowered positions fordetermining corresponding raised and lowered position of said first armstructure.
 6. A screw printing machine as claimed in claim 5, whereinsaid height adjustment mechanism includes a threaded spindle carryingsaid manually adjustable member at one end thereof and said piston andcylinder device includes a threaded portion in threaded engagement withsaid threaded spindle, whereby rotation of said manually adjustablemember is effective to displace said piston and cylinder device and,thereby, to adjust the height of said stop.
 7. A screen printing machineas claimed in claim 6, wherein said height adjustment mechanismcomprises a wedge co-operating with said stop and a wedge adjustmentmechanism for displacing said wedge relative to said stop so as tocorrespondingly vary the location of said stop, said wedge adjustmentmechanism comprising a piston and cylinder device operable to displacesaid wedge to and fro and thereby to displace said stop between raisedand lowered positions, and an adjustable connection between saidmanually adjustable member and said piston and cylinder device andoperable to displace said piston and cylinder device and therewith saidwedge member in response to adjustment of said manually adjustablemember.
 8. A screen printing machine as claimed in claimed 4, whereinsaid height adjustment mechanism comprises a wedge co-operating withsaid stop and said manually adjustable member forms part of a wedgeadjustment mechanism for displacing said wedge relative to said stop soas to correspondingly vary the vertical location of said stop.
 9. Ascreen printing machine, comprising:a support structure for supportingsaid machine on a floor; said support structure comprising a verticalsupport having a vertical axis; a first arm structure; a bearingarrangement co-axial with said vertical cylindrical support andsupporting said first arm structure for rotation about the verticalaxis; said first arm structure comprising a plurality of first armsequiangularly distributed about the vertical axis; said first armsextending radially outwardly of said vertical axis and having free outerends; printing screen support platens carried on said free outer ends ofsaid first arms; a second arm structure mounted on said vertical supportabove said first arm structure; said second arm structure comprising aplurality of second arms equiangularly distributed about said verticalaxis; said second arms extending radially outwardly of said verticalaxis above said first arm structure and having free outer ends; saidsecond arms carrying, at said free outer ends thereof, squeegeemechanisms and printing screen supports for holding printing screensbelow said squeegee mechanisms; an indexing mechanism connected betweensaid first arm structure and said support structure and operable torotate said first arm structure stepwise around the vertical axis so asto position said printing screen support platens in succession inregistry with said squeegee mechanisms; a lifting device connectedbetween said first arm structure and said support structure and operableto displace the former to and from a raised position; interengageablepairs of connectors provided on said first and second arms,respectively, at locations adjacent and radially inward of said printingscreen supports and operable to forcibly connect said first arms to saidsecond arms on displacement of said first arm structure into the raisedposition, and thereby to counteract bending of said arms during theoperation of said squeegee mechanisms; a shock absorber connectedbetween said support structure and said indexing mechanism; and saidshock absorber comprising a cylinder, an elongate piston slidable to andfro in said cylinder and a plurality of fluid outlet openings spacedapart along said cylinder for permitting hydraulic fluid to escape fromsaid cylinder in response to sliding of said piston within said cylindertowards one end of said cylinder, and said piston being dimensioned tosuccessively block said outlet openings as said piston slides into saidcylinder.
 10. A screen printing machine as claimed in claim 9, whereinsaid outlet openings are distributed over a predetermined length of saidcylinder and said piston has a diameter substantially equal to theinternal diameter of said cylinder so that said piston cansimultaneously block said outlet openings, said cylinder including afurther outlet openings at said one end of said cylinder.
 11. A screenprinting mechanism as claimed in claim 10, wherein said cylinder is aninner cylinder and said shock absorber further comprises an outercylinder, said inner cylinder fitting snugly into said outer cylinder,and further comprising seals between said inner and outer cylinders atopposite ends thereof, said outer cylinder having an outlet for thehydraulic fluid and said inner cylinder having an external recesscommunicating with said outlet openings in said inner cylinder and withsaid outlet in said outer cylinder.
 12. A screen printing machine,comprising:a support structure for supporting said machine on a floor;said support structure comprising a vertical support having a verticalaxis; a first arm structure; a bearing arrangement co-axial with saidvertical cylindrical support and supporting said first arm structure forrotation about the vertical axis; said first arm structure comprising aplurality of first arms equiangularly distributed about the verticalaxis; said first arms extending radially outwardly of said vertical axisand having free outer ends; printing screen support platens carried onsaid free outer ends of said first arms; a second arm structure mountedon said vertical support above said first arm structure; said second armstructure comprising a plurality of second arms equiangularlydistributed about said vertical axis; said second arms extendingradially outwardly of said vertical axis above said first arm structureand having free outer ends; said second arms carrying, at said freeouter ends thereof, squeegee mechanisms and printing screen supports forholding printing screens below said squeegee mechanisms; an indexingmechanism connected between said first arm structure and said supportstructure and operable to rotate said first arm structure stepwisearound the vertical axis so as to position said printing screen supportplatens in succession in registry with said squeegee mechanisms; alifting device connected between said first arm structure and saidsupport structure and operable to displace the former to and from araised position; interengageable pairs of connectors provided on saidfirst and second arms, respectively, at locations adjacent and radiallyinward of said printing screen supports and operable to forcibly connectsaid first arms to said second arms on displacement of said first armstructure into the raised position, and thereby to counteract bending ofsaid arms during the operation of said squeegee mechanisms; and saidindexing mechanism comprising an indexing arm mounted for pivotationabout said vertical support and having a free end, a piston and cylinderdevice connected between said indexing arm and said support structurefor effecting pivotation of said indexing arm; a connector member at thefree end of said indexing arm and an actuator on said free end of saidindexing arm for raising said connector member into drive transmittingengagement with successive ones of said first arms.
 13. A screenprinting machine, comprising:a support structure for supporting saidmachine on a floor; said support structure comprising a vertical supporthaving a vertical axis; a first arm structure; a bearing arrangementco-axial with said vertical cylindrical support and supporting saidfirst arm structure for rotation about the vertical axis; said first armstructure comprising a plurality of first arms equiangularly distributedabout the vertical axis; said first arms extending radially outwardly ofsaid vertical axis and having free outer ends; printing screen supportplatens carried on said free outer ends of said first arms; a second armstructure mounted on said vertical support above said first armstructure; said second arm structure comprising a plurality of secondarms equiangularly distributed about said vertical axis; said secondarms extending radially outwardly of said vertical axis above said firstarm structure and having free outer ends; said second arms carrying, atsaid free outer ends thereof, squeegee mechanisms and printing screensupports for holding printing screens below said squeegee mechanisms; anindexing mechanism connected between said first arm structure and saidsupport structure and operable to rotate said first arm structurestepwise around the vertical axis so as to position said printing screensupport platens in succession in registry with said squeegee mechanisms;a lifting device connected between said first arm structure and saidsupport structure and operable to displace the former to and from araised position; interengageable pairs of connectors provided on saidfirst and second arms, respectively, at locations adjacent and radiallyinward of said printing screen supports and operable to forcibly connectsaid first arms to said second arms on displacement of said first armstructure into the raised position, and thereby to counteract bending ofsaid arms during the operation of said squeegee mechanisms; saidprinting screen supports each comprising a screen holder for holding aprinting screen; a support arrangement carrying said screen holder; andan adjustment mechanism between said support arrangement and therespective one of said second arms, said adjustment mechanism comprisingmeans for adjustably displacing said support arrangement longitudinallyand transversely of its respective second arm; said screen holdercomprising a pair of spaced screen supports for supporting oppositeedges of the printing screen; and said support arrangement beingprovided with adjustable connectors between said screen supports andsaid support arrangement to allow said screen supports to be adjusted inposition relative to one another and to said support arrangement.
 14. Ascreen printing machine as claimed in claim 13, wherein said supportarrangement comprises a pair of parallel, substantially horizontalelongate members extending along the respective one of said second armsand said adjustable connectors are adjustably slidable along saidelongate members.
 15. A screen printing machine as claimed in claim 14,wherein said elongate members comprise opposite sides of a supportframe, and said adjustment mechanism comprises means for adjusting saidsupport frame angularly relative to the respective one of said secondarms.
 16. A screen printing machine as claimed in claim 13, whereinadjusting means comprise a pair of adjustment members spaced apart alongthe respective second arm and operable to displace said screen supporttransversely of the respective second arm and a further adjustmentmember operable to displace said screen support longitudinally of therespective second arm.
 17. A screen printing machine, comprising:asupport structure for supporting said machine on a floor; said supportstructure comprising a vertical support having a vertical axis; a firstarm structure; a bearing arrangement co-axial with said verticalcylindrical support and supporting said first arm structure for rotationabout the vertical axis; said first arm structure comprising a pluralityof first arms equiangularly distributed about the vertical axis; saidfirst arms extending radially outwardly of said vertical axis and havingfree outer ends; printing screen support platens carried on said freeouter ends of said first arms; a second arm structure mounted on saidvertical support above said first arm structure; said second armstructure comprising a plurality of second arms equiangularlydistributed about said vertical axis; said second arms extendingradially outwardly of said vertical axis above said first arm structureand having free outer ends; said second arms carrying, at said freeouter ends thereof, squeegee mechanisms and printing screen supports forholding printing screens below said squeegee mechanisms; an indexingmechanism connected between said first arm structure and said supportstructure and operable to rotate said first arm structure stepwisearound the vertical axis so as to position said printing screen supportplatens in succession in registry with said squeegee mechanisms; alifting device connected between said first arm structure and saidsupport structure and operable to displace the former to and from araised position; interengageable pairs of connectors provided on saidfirst and second arms, respectively, at locations adjacent and radiallyinward of said printing screen supports and operable to forcibly connectsaid first arms to said second arms on displacement of said first armstructure into the raised position, and thereby to counteract bending ofsaid arms during the operation of said squeegee mechanisms; and saidsecond arms each comprising a first arm member extending radiallyoutwardly of said vertical support, a second arm member extendingradially outwardly of said vertical support and downwardly and outwardlyinclined towards said first arm member, releasable fasteners forsecuring said second arm member to said first arm member and anadjustment mechanism connected between said first and second arm membersand adjustable to displace said second arm member in position along saidfirst arm member while said fasteners are released.
 18. A screenprinting machine, comprising:a support structure for supporting saidmachine on a floor; said support structure comprising a vertical supporthaving a vertical axis; a first arm structure; a bearing arrangementco-axial with said vertical cylindrical support and supporting saidfirst arm structure for rotation about the vertical axis; said first armstructure comprising a plurality of first arms equiangularly distributedabout the vertical axis; said first arms extending radially outwardly ofsaid vertical axis and having free outer ends; printing screen supportplatens carried on said free outer ends of said first arms; a second armstructure mounted on said vertical support above said first armstructure; said second arm structure comprising a plurality of secondarms equiangularly distributed about said vertical axis; said secondarms extending radially outwardly of said vertical axis above said firstarm structure and having free outer ends; said second arms carrying, atsaid free outer ends thereof, squeegee mechanisms and printing screensupports for holding printing screens below said squeegee mechanisms; anindexing mechanism connected between said first arm structure and saidsupport structure and operable to rotate said first arm structurestepwise around the vertical axis so as to position said printing screensupport platens in succession in registry with said squeegee mechanisms;a lifting device connected between said first arm structure and saidsupport structure and operable to displace the former to and from araised position; interengageable pairs of connectors provided on saidfirst and second arms, respectively, at locations adjacent and radiallyinward of said printing screen supports and operable to forcibly connectsaid first arms to said second arms on displacement of said first armstructure into the raised position, and thereby to counteract bending ofsaid arms during the operation of said squeegee mechanisms; and saidsqueegee mechanisms each comprising a squeegee which is slidable incontact with one of the printing screens; a carriage movable along arespective one of said second arms and carrying said squeegee formovement to and fro over the printing screen, said squeegee having apair of mutually angularly disposed squeegee blades, and a pivotalconnection between said squeegee and said carriage, said pivotalconnection permitting pivotation of said squeegee about an axis parallelto said squeegee blades for bringing said squeegee blades alternatelyinto contact with said printing screen during the to and fro moments,respectively, of said carriage; a piston and cylinder device mounted onsaid carriage and connected to said carriage and said squeegee, saidpiston and cylinder device being operable to pivot said squeegee aboutsaid axis, and an adjustable stop mechanism adjustably limiting thepivotation of said squeegee by said piston and cylinder device.
 19. Ascreen printing machine as claimed in claim 18, wherein said piston andcylinder device comprises a piston rod connected to said squeegee and acylinder connected to said carriage and wherein said adjustable stopmechanism comprises an elongate member extending parallel to said pistonand cylinder device and connected to said piston rod for movementtherewith, an abutment fixed relative to said cylinder and a pair ofstop members carried by said elongate member and adjustable in positionalong said elongate member, said stop members being movable into contactwith said abutment, in response to extension and retraction of saidpiston rod, so as to limit the range of movement of said piston rod. 20.A screen printing machine, comprising:a support structure for supportingsaid machine on a floor; said support structure comprising a verticalsupport having a vertical axis; a first arm structure; a bearingarrangement co-axial with said vertical cylindrical support andsupporting said first arm structure for rotation about the verticalaxis; said first arm structure comprising a plurality of first armsequiangularly distributed about the vertical axis; said first armsextending radially outwardly of said vertical axis and having free outerends; printing screen support platens carded on said free outer ends ofsaid first arms; a second arm structure mounted on said vertical supportabove said first arm structure; said second arm structure comprising aplurality of second arms equiangularly distributed about said verticalaxis; said second arms extending radially outwardly of said verticalaxis above said first arm structure and having free outer ends; saidsecond arms carrying, at said free outer ends thereof, squeegeemechanisms and printing screen supports for holding printing screensbelow said squeegee mechanisms; an indexing mechanism connected betweensaid first arm structure and said support structure and operable torotate said first arm structure stepwise around the vertical axis so asto position said printing screen support platens in succession inregistry with said squeegee mechanisms; a lifting device connectedbetween said first arm structure and said support structure and operableto displace the former to and from a raised position; interengageablepairs of connectors provided on said first and second arms,respectively, at locations adjacent and radially inward of said printingscreen supports and operable to forcibly connect said first arms to saidsecond arms on displacement of said first arm structure into the raisedposition, and thereby to counteract bending of said arms during theoperation of said squeegee mechanisms; and said squeegee mechanismincluding a squeegee which is slidable in contact with one of theprinting screens, a carriage movable along a respective one of saidsecond arms and carrying said squeegee for movement to and fro over saidprinting screen and a vertically adjustable connection between saidsqueegee and said carriage, said vertically adjustable connectioncomprising a cylindrical threaded member extending downwardly from saidcarriage, a manually rotatable adjustment ring member in threadedengagement with said cylindrical threaded member, and a slide membercarrying said squeegee, said slide member being supported on said ringmember for vertical displacement upon rotation of said ring member aboutsaid cylindrical threaded member.
 21. A screen printing machine,comprising:a support structure for supporting said machine on a floor;said support structure comprising a vertical support having a verticalaxis; a first arm structure; a bearing arrangement co-axial with saidvertical cylindrical support and supporting said first arm structure forrotation about the vertical axis; said first arm structure comprising aplurality of first arms equiangularly distributed about the verticalaxis; said first arms extending radially outwardly of said vertical axisand having free outer ends; printing screen support platens carried onsaid free outer ends of said first arms; a second arm structure mountedon said vertical support above said first arm structure; said second armstructure comprising a plurality of second arms equiangularlydistributed about said vertical axis; said second arms extendingradially outwardly of said vertical axis above said first arm structureand having free outer ends; said second arms carrying, at said freeouter ends thereof, squeegee mechanisms and printing screen supports forholding printing screens below said squeegee mechanisms; an indexingmechanism connected between said first arm structure and said supportstructure and operable to rotate said first arm structure stepwisearound the vertical axis so as to position said printing screen supportplatens in succession in registry with said squeegee mechanisms; alifting device connected between said first arm structure and saidsupport structure and operable to displace the former to and from araised position; interengageable pairs of connectors provided on saidfirst and second arms, respectively, at locations adjacent and radiallyinward of said printing screen supports and operable to forcibly connectsaid first arms to said second arms on displacement of said first armstructure into the raised position, and thereby to counteract bending ofsaid arms during the operation of said squeegee mechanisms; said screenprinting machine further comprising: suspension arrangements supportingsaid squeegee mechanisms from said second arms for movement to and froover the printing screens; piston and cylinder devices connected betweensaid suspension arrangements and said first arms for effecting the toand for movement of said squeegee mechanisms; and air brakes for brakingthe to and fro movement of said squeegee mechanisms, said piston andcylinder devices including piston rods extending through said air brakesand said air brakes each comprising a pair of cylindrically curved brakemembers at opposite sides of a respective one of said piston rods andair passages for supplying compressed air to said brake members so as tourge said brake members against the opposite sides of respective one ofsaid piston rods; a stop mounted on said support structure for limitingthe upward movement of said first arm structure towards said second armstructure; a height adjustment mechanism connected to said stop foradjustably raising and lowering said stop and thereby adjusting thelimit of the upward movement of said first arm structure; and saidheight adjustment mechanism including a manually adjustable member,which is readily accessible from the exterior of said screen printingmachine, for operating said height adjustment mechanism; a shockabsorber connected between said support structure and said indexingmechanism; said shock absorber comprising a cylinder, an elongate pistonslidable to and fro in said cylinder and a plurality of fluid outletopenings spaced apart along said cylinder for permitting hydraulic fluidto escape from said cylinder in response to sliding of said pistonwithin said cylinder towards one end of said cylinder, and said pistonbeing dimensioned to successively block said outlet openings as saidpiston slides into said cylinder; said indexing mechanism comprising anindexing arm mounted for pivotation about said vertical support andhaving a free end, a piston and cylinder device connected between saidindexing arm and said support structure for effecting pivotation of saidindexing arm; a connector member at the free end of said indexing armand an actuator on said free end of said indexing arm for raising saidconnector member into drive transmitting engagement with successive onesof said first arms; said printing screen supports each comprising ascreen holder for holding a printing screen; a support arrangementcarrying said screen holder; and an adjustment mechanism between saidsupport arrangement and the respective one of said second arms, saidadjustment mechanism comprising means for adjustably displacing saidsupport arrangement longitudinally and transversely of its respectivesecond arm; said screen holder comprising a pair of spaced screensupports for supporting opposite edges of the printing screen; saidsupport arrangement being provided with adjustable connectors betweensaid screen supports and said support arrangement to allow said screensupports to be adjusted in position relative to one another and to saidsupport arrangement; said second arms each comprising a first arm memberextending radially outwardly of said vertical support, a second armmember extending radially outwardly of said vertical support anddownwardly and outwardly inclined towards said first arm member,releasable fasteners for securing said second arm member to said firstarm member and an adjustment mechanism connected between said first andsecond arm members and adjustable to displace said second arm member inposition along said first arm member while said fasteners are released;said squeegee mechanisms each comprising a squeegee which is slidable incontact with one of the printing screens; a carriage movable along arespective one of said second arms and carrying said squeegee formovement to and fro over the printing screen, said squeegee having apair of mutually angularly disposed squeegee blades, and a pivotalconnection between said squeegee and said carriage, said pivotalconnection permitting pivotation of said squeegee about an axis parallelto said squeegee blades for bringing said squeegee blades alternatelyinto contact with said printing screen during the to and fro moments,respectively, of said carriage; a piston and cylinder device mounted onsaid carriage and connected to said carriage and said squeegee, saidpiston and cylinder device being operable to pivot said squeegee aboutsaid axis, and an adjustable stop mechanism adjustably limiting thepivotation of said squeegee by said piston and cylinder device; and saidsqueegee mechanism including a squeegee which is slidable in contactwith one of the printing screens, a carriage movable along a respectiveone of said second arms and carrying said squeegee for movement to andfro over said printing screen and a vertically adjustable connectionbetween said squeegee and said carriage, said vertically adjustableconnection comprising a cylindrical threaded member extending downwardlyfrom said carriage, a manually rotatable adjustment ring member inthreaded engagement with said cylindrical threaded member, and a slidemember carrying said squeegee, said slide member being supported on saidring member for vertical displacement upon rotation of said ring memberabout said cylindrical threaded member.